Network fault recovery method and apparatus

ABSTRACT

The present invention provides layer one, two and three (L 1 /L 2 /L 3 ) Integration and L 1  cut-through path utilization in an apparatus and method of fault recovery. A switch combines an IP router with L 2  capabilities, and an L 1  cross connect (optical or electrical). A network of such switches is configured with label switched paths (LSP) that correspond to layer  1  (L 1 ) cut-through paths. The layer  2  (L 2 ) cut-through path is over laid on the L 1  cut-through path and the L 2  cut-through path is used for IP data flows. Preferably, the L 2  cut-through paths are defined as label switched paths (LSP) and the L 1  cut-through paths are each an end-to-end path established with L 1  cross connects associated with each switch.

FIELD OF THE INVENTION

The present invention relates to network fault recovery method andapparatus and is particularly concerned with recovery at higher layersfrom physical layer faults.

BACKGROUND OF THE INVENTION

Currently, the traffic reliability of large telecommunications networkssuch as core networks used for Internet service providers (ISPs) or formajor corporate backbones is dependent upon the traffic protectionresources built into the network elements. To ensure that the desiredavailability of network connections is maintained and protected, it isstandard practice in the telecommunications industry to rely on routingalgorithms for handling link or equipment failures. However, with atypical failure reaction time of 30 seconds, conventional routingprotocols are inherently too slow for today's high speed networks. Thisresults in inappropriate transmission down time, particularly for videoand voice transmission.

A faster solution conventionally used to protect network connectionsconsists of implementing protection in the physical layer (layer 1) ofthe network by installing redundant equipment so that if one physicallink fails, another can rapidly be switched into place.

By contrast to relying on the routing protocols for protecting theavailability of network connections, the installation of redundantequipment results in a much faster failure reaction time which, forexample in SONET rings is usually in the neighbourhood of 50milliseconds.

Redundancy of equipment has long been accepted by carrier grade networksas a way to ensure availability and reliability. However networks notrequiring carrier grade protection, still desire rapid recovery fromphysical failures, particularly in high throughput links such as carriedin optical fiber, e.g. OC-192.

However, the use of redundant layer 1 equipment for protection presentsa number of disadvantages. First, more network links must be installed.For example, current protection configurations which require theinstallation of additional fiber links between network nodes includededicated protection (1 protection fiber for each fiber link alsoreferred to as 1:1 protection), shared protection (1 protection fiberfor N fiber links or 1:N protection) and ring protection.

The accommodation of multiple fiber links necessitates replicating someof the equipment relating to optical link budgets at each network node.Duplicating this equipment may prove to have a major impact on theoverall cost of the network.

In addition to the high cost associated with installing additionalequipment for traffic protection, another drawback of the use ofredundant layer 1 equipment is that the additional bandwidth capacitycreated therefrom is exclusively dedicated to traffic protection andremains unused, or is pre-emptable, in the absence of network failures.This increases the cost of the bandwidth.

In view of the slow reaction time of the routing protocols, the highcost and the inefficient bandwidth management associated with the use ofadditional layer 1 equipment, it is desirable to provide acost-effective and efficient protection mechanism which providesadequate reaction time to failures and maximizes the utilization of theavailable resources present in the network.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved networkfault recovery method and apparatus.

In accordance with the present invention L1/L2/L3 Integration and L1cut-through path utilization are provided in an apparatus and method offault recovery.

In accordance with an aspect of the present invention there is provideda switch which combines an IP router with L2 capabilities, and an L1cross connect (optical or electrical).

In accordance with another aspect of the invention there is provided anetwork in which switches are configured with label switched paths(LSPS) that correspond to layer 1 (L1) cut-through paths.

Conveniently, a layer 2 (L2) cut-through path is over laid on the L1cut-through path and the L2 cut-through path is used for IP data flows.

Preferably, the L2 cut-through paths are defined as label switched paths(LSPs). And the L1 cut-through paths are each an end-to-end pathestablished with L1 cross connects associated with each switch.

In accordance with another aspect of the present invention a method isprovided in which upon failure of a physical link, all LSP endpointsassociated with affected L1 cut-through paths are notified by physicaldetection methods.

Preferably, label switch paths are defined corresponding to a respectiveL1 cut-through path, the MPLS entity managing an LSP is notified of LSPfailures that correspond to L1 cut-through path failure, and backupprocedures are then executed to restore IP forwarding.

According to an aspect of the present invention there is provided amethod of fault recovery for a network including the steps ofestablishing a physical topology for the network, aligning a logicaltopology for the network with the physical topology, and using a faultindication from the physical topology to effect fault recovery in thelogical topology.

In accordance with another aspect of the present invention there isprovided an apparatus for data networking comprising a cross connect forswitching at a physical layer, a router for redirecting data packets ata logical layer coupled to the cross connect, and a fault recoverymechanism responsive to a fault indication in the physical layer foreffecting a recovery in the logical layer.

Conveniently, the router includes an internetworking protocol (IP).

Preferably, the internetworking protocol includes multi-protocol labelswitching (MPLS).

In accordance with another aspect of the present invention there isprovided a network comprising a plurality of nodes, each node includinga cross connect for switching at a physical layer, a router forredirecting data packets at a logical layer coupled to the cross connectand a fault recovery mechanism responsive to a fault indication in thephysical layer for effecting a recovery in the logical layer, aplurality of physical connections between nodes via the respective crossconnects, a plurality of logical routes between nodes via the respectiverouters, and an alternative logical route for use by the fault recoverymechanism.

In accordance with another embodiment of the present invention there isprovided in a network including a plurality of nodes and having aplurality of communications layers, a method of providing fault recoverycomprising the steps of aligning at least a first and second layer ofthe plurality of communications layers, for a given path in the firstlayer, defining a corresponding path in the second layer and analternative path in the second layer, the alternative path in the secondlayer corresponding to an alternative path in the first layer disjointfrom the given path, and on detection in the first layer of a fault inthe given path, switching in the second layer from the correspondingpath to the alternative path, whereby fault recovery in the network isprovided

Advantages of the present invention include faster recovery from layer 1failure than provided by L3 routing algorithms and integration of thelayers 1, 2 and 3 networks into a common topology (a network managementsimplification and potential equipment cost saving).

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be further understood from the followingdetailed description, with reference to the drawings in which:

FIG. 1 illustrates a known ATM link between two label switched routers(LSR);

FIGS. 2a) and b) illustrate a network of four routers showing topologyand label switched paths respectively;

FIGS. 3a), b) and c) illustrate a physical topology, L3 links and OSPFtopology, respectively;

FIG. 4 illustrates connectionless layer 3 internet protocol (IP)forwarding in a network of four routers;

FIG. 5 illustrates label switching in a network of four label switchingrouters;

FIGS. 6a), b) and c) illustrate routers on SONET ring and how they aretypically connected;

FIGS. 7a), b), and c) illustrate routers in a TDM overlay;

FIGS. 8a), b), and c) illustrate a switch in accordance with a firstembodiment of the present invention and a physical and logicaltopologies for two such switches;

FIGS. 9a) and b) illustrate an exemplary network's physical and routertopologies made up of switches of FIG. 8;

FIGS. 10a) and b) illustrate the network of FIG. 9a) and b) showing alayer 1 (L1) cut-through path. In the network of FIG. 10, a layer 1cross connected path is treated as a layer 1 cut-through path by therouters;

FIGS. 11a) and b) illustrate IP packet forwarding using the L1cut-through path of FIG. 10b);

FIGS. 12a) and b) illustrate the effect of an L1 link failure on the useof layer 1 cut-through path by L3 forwarding;

FIG. 13 illustrates a series of L1 cut-through paths based on the FIG.10 topology;

FIGS. 14a) and b) illustrate an L1 failure in the network of FIG. 13,and the L3 routing view from the point of router R5;

FIGS. 15a) and b) illustrate the network topology of FIG. 14a) showingonly the L1 cut-through path not affected by the failed link and an LSPset up using the L1 cut-through path, respectively;

FIGS. 16a) and b) illustrate routing tables, label tables andcross-connects for the topology of FIG. 15b), and illustrate how data isforwarded on a recovery LSP that uses an L1 cut-through path;

FIG. 17 illustrates an L3 stabilised topology database view after thefailure has been used to update the L3 routing tables throughout thenetwork;

FIG. 18 illustrates how a router recovers from the failure of a secondL1 cut-through path affected by the failure of FIG. 14;

FIGS. 19a) and b) illustrate a network topology when the failed L1 linkrecovers and L1 cut-through paths are automatically re-established byoriginal configuration information, respectively.

DETAILED DESCRIPTION

Referring to FIG. 1, there is illustrated a known ATM link between twolabel switched routers (LSR). An ATM link 10 runs between LSR 12 andLSR14 and as shown in the expanded link section 16, an ATM linktypically carries both connectionless traffic 18 and connection orientedtraffic such as MPLS label switched paths 20 and 22. In IP routernetworks, control and data planes are typically not separated.

IP Control Traffic consists of:

Routing protocol messages such as OSPF Hello, OSPF Link StateAdvertisements

L3 to L2 Address resolution (ARP), flow control (ICMP)

Many other protocols (traceroute, ping, multicast)

IP Data Traffic consists of:

Host-to-host data exchanged via various TCP and UDP protocols (e.g.,file transfer with FTP)

Network-to-network data which is carried in TCP or UDP packets (e.g.,BGP4 updates)

Typical router-router links 10 carry both control and data traffic, 18.It is possible to separate IP control and data streams. This could be onseparate links or could be on separate channels within a channeled linklike ATM.

Separation of control and data is seen in MPLS where:

IP routing control is done in a connectionless manner

IP data can be forwarded on Label Switched Paths that are in differentchannels than IP connectionless control.

Referring to FIGS. 2a) and b) there are illustrated a network of fourrouters showing router topology and label switched paths, respectively.In FIG. 2a) routers 30, 32, 34 and 36 are connected by: physical link 38between routers 30 and 32; physical link 40 between routers 30 and 34;and physical link 42 between routers 30 and 36. In FIG. 2b) routers 30,32, 34 and 36 are interconnected by: MPLS label switched path (LSP) 50between routers 30 and 32; LSP 52 between routers 30 and 34; LSP 54between routers 30 and 36; LSP 56 between routers 32 and 34; LSP 58between routers 32 and 36; and LSP 60 between routers 34 and 36.

In MPLS, the separation of planes is useful in decoupling the number ofIGP (Interior Gateway Protocol ) links from the number of forwardinglinks in the network.

The Interior Gateway Protocol (i.e., a routing protocol) links carrycontrol traffic for the IGP. Usually the physical topology and the IGPtopology coincide as shown in FIG. 2a). MPLS label switched paths arecreated over physical links to form high mesh connectivity for dataforwarding as shown in FIG. 2b).

Referring to FIGS. 3a), b) and c) there are illustrated a physicaltopology, links topology and OSPF topology database view, respectively.

In FIG. 3a) routers 30, 32, 34 and 36 are connected by: physical link 62between routers 30 and 32; physical link 64 between routers 30 and 34;physical link 66 between routers 30 and 36; and physical link 68 betweenrouters 32 and 34. In FIG. 3b) routers 30, 32, 34 and 36 areinterconnected by links: OSPF link 70 between routers 30 and 32; OSPFlink 72 between routers 30 and 34; OSPF link 74 between routers 30 and36; and static route 76 between routers 32 and 34. In FIG. 3c) the OSPFtopology database view of the network consists of: link 80 betweenrouters 30 and 32; link 82 between routers 32 and 36; and link 84between routers 34 and 36.

In such a network, control and data streams can be separated ontodifferent links. For example, a link 76 between two routers 32 and 34 isused only to carry traffic for static IP routes. No IP routing controltraffic passes over this link. However it is not possible to completelyseparate control and data in this example as other IP control packetscould still use the link 76 (e.g., ping).

Currently, L2 and L3 control and data technologies are being combinedinto IP routers that incorporate switching technologies like ATM. Thishas made an impact on how packets are forwarded. To understand thisimpact, L3 forwarding is reviewed.

Referring to FIG. 4 there is illustrated a connectionless layer 3internet protocol (L3 IP) forwarding in a network of four routers. Thenetwork includes routers 88, 90, 92 and 94. For the purposes ofdiscussion and simplification only routing over two links, A link 96 andB link 98 are described.

L3 Forwarding takes IP packets, for example packet 100 and performs alookup on the destination IP address in an IP forwarding table (102,104, 106), for example R1 table 102 shows link 94, as the next hop. Thepacket is sent on link A and arrives at router R2 where another lookupon the destination IP address occurs in R2 table 104. The result of thatlook up is B link, 98, as next hop. A successful lookup results in anidentifier for an outgoing link on which to place the packet. This isrepeated at each router until a router is reached which directlysupports the destination IP address.

In combined L2/L3 switches, instead of forwarding all IP packets in ahop-by-hop connectionless manner, MPLS and other schemes use additionsto IP routing control to leverage L2 forwarding for IP packets. This hasseveral advantages including simplicity of the forwarding operation, andthe ability to have packets flow along arbitrary paths (as opposed tojust shortest). Multi-Protocol Label Switching MPLS provides a method ofsetting up L2 forwarding in these switches.

Referring to FIG. 5 there is illustrated a label switched router in anetwork of four label switched routers. For the network of FIG. 4 alabel switched path 108 is defined over links 96, 97, and 99. Note thatthis is not a path that would have been chosen by the shortest pathalgorithm of an L3 routing protocol like OSPF (assuming each link wasequal cost). This constraint is not necessarily imposed on MPLS LSPs.

In operation, after MPLS Label Switched Paths (LSP) are set up, forexample LSP 108, IP packets 110 at the start of an LSP undergo an L3lookup as part of L3 forwarding e.g., table 102. If they match aForwarding Equivalence Class (FEC), they are sent to the correspondingLSP 108. An MPLS label is added to the packet 112 and it is sent out alink with this encapsulation. At the next MPLS Label Switched Router(LSR), a label swap occurs in a L2 forwarding table 114 (MPLS IncomingLabel Map).

MPLS Forwarding Example:

LSP defined over links 96, 97, 99

Packet 110 arriving at R1 88 destined for R4 94 is handled by L3forwarding 102 and placed on an LSP 108. At R2 90 and R3 92, forwardingis handled by L2 forwarding tables 114 and 116, respectively (i.e.,switching, also called label swapping in this context). At R4 94, thefinal L2 label lookup occurs as this is the end of the LSP. The packetis decapsulated from the MPLS label and passed to the router IP stackfor further processing.

This is sometimes called “route once, switch many”.

Referring to FIG. 6 there is illustrated an example of a typicalnetwork. FIG. 6a) illustrates a physical topology for a SONET ring withattached routers. Routers are connected to Add/Drop Muxes (ADMs) aroundring. FIG. 6b) illustrates a typical logical full-mesh router topologyconfigured on the SONET ring. Router networks use link facilities thatare paths in an underlying L1 physical network. Some of these pathsbypass other routers. If multiple router-router links share the samephysical segment at some point, they will all go down if that segmentfails. FIG. 6c) illustrates the effect of a SONET ring segment failureon the configured router-router links. Networks using such L1 facilitiesusually request physical diversity in their link service. However, thisis not always possible, for example, routers connected over anunprotected SONET ring. With multiple L3 link failures, it can takeseconds for the routing protocol to recover IP forwarding. L3 traffic isheld up until shortest paths re-established even though bandwidth andconnectivity may exist. Thus the effect of a single link failure in L1can have large impacts on the L3 topology and the time it takes torecover connectionless forwarding.

In a network with TDM switches in Layer 1, routers are similarlyconnected as. SONET networks. Specifically, cross-connect paths aredefined for router-router links. In FIG. 7a), TDM switches S1, S2, S3form the L1 physical network with 3 physical links. Routers use TDMpaths setup over those switches and the logical router topology in FIG.7. b) shows 5 links. When physical link S1-S2 fails, two router-routerlinks are affected (FIG. 7c) because they shared that L1 link for acommon portion of their cross connect paths.

DETAILED DESCRIPTION OF EMBODIMENT OF PRESENT INVENTION

Referring to FIGS. 8a), b), and c) there is illustrated a switch inaccordance with a first embodiment of the present invention and physicaland logical topologies for two such switches. In FIG. 8a) a switch 150combines an IP label switching router 152 with a layer one (L1) crossconnect 154. The switch 150 is defined as a switch that is a traditionalIP router 152 (with some L2 switching) linked with an L1multiplexor/demultiplexor and cross connect 154. For example:

an IP label switching router and a SONET ADM

an IP label switching router and a TDM cross connect

In this combined switch, traffic can enter the cross connect and pass upto the router where it is forwarded onto another outgoing channel in thecross connect. Traffic can also enter the cross connect and exit withoutpassing up to the router.

FIG. 8b) illustrates a simple network of two switches 160 and 162 ofFIG. 8a) connected together by physical link 164, allocating one or morechannels to connect the routers on either end of the link. FIG. 8c)illustrates the resulting logical connection between the router portionsof the switches of FIG. 8b).

Referring to FIGS. 9a) and b) there are illustrated an exemplarynetwork's physical and router topologies made up of switches of FIG. 8.Integration of L1, L2, and L3 is achieved, i.e., an L1/L2/L3 network isestablished in the following way.

1. Define an IP network with many interconnected L1/L2/L3 switches. NoL1 restoration mechanisms are needed or assumed (e.g., SONET restoral).

2. L3/L2/L1 topologies are aligned. All router-router links are now onephysical hop and routers view the physical topology. This networkdiffers from networks where L1 and L2 are separated in that no L1 paths(series of cross connected channels) are used as router-router links.

For L3 forwarding in L1/L2/L3 Network, connectionless forwarding ofpackets traverses only direct physical links on the router-routerchannels of those links. L3 connectionless traffic may traverse manyhops, e.g., packets from R8 to R3 would traverse R8-R7-R1-R2-R3 in FIG.9b).

Referring to FIGS. 10a) and b), there is illustrated the network of FIG.9a) and b) showing a layer 1 (L1) cut-through path. In the network ofFIG. 10, an existing use of the L1 path between R8 and R5 would be as arouter-router link which carries IP control and data traffic. This isnot how this L1 path is used in the present embodiment of thisinvention.

An L1 cut-through path is illustrated in FIG. 10a). Routers194,196,198,200, 202, and 204 are each connected to respective add/dropMUXs (ADM) 184, 186, 206, 208, 210, and 212 in a SONET ring 214 withoutprotection. Routers 190 and 192 with TDM fabrics 180 and 182 are linkedto ADMs 184 and 212. An L1 cut-through path 170 is defined throughcross-connects 180, 182, 184, and 186 associated with routers R8 190, R7192, R6 194, R5 196.

This is equivalent to a private line between routers R8 and R5.

Other cut-through paths can be defined over shared physical links.

A L1 cut-through path 170 is established as follows:

1) Define a L1 cut-through path 170 that includes normal connections inL1 networks and consists of channels in links 172, 174, and 176concatenated at cross connect points 180, 182, 184, and 186.

2) Let routers at the L1 cut-through path end points (190 and 196) viewL1 cut-through paths as valid next hops available for use in the IPForwarding table, and not as a router-router links that pass IGP controltraffic.

There are two possible ways for the router to view the L1 cut-throughpath as statically routed links, or, as if they were an L2 switched path(like an MPLS LSP). For example, in the optical domain, paths thatbypass SONET boxes are like static LSPs. That is, Label DistributionProtocol cannot create them, and in the optical topology they areanalogous to PVCs in an ATM topology.

In a preferred embodiment the router views the L1 cut-through path as anMPLS Label Switched Path. Existence and use of L1 cut-through paths doesnot preclude the use of MPLS dynamic LSPs. Routers are not aware of L1cut-through paths that tandem through them (e.g., optical bypass inSONET). In FIG. 10b), router R6 is unaware of the L1 cut-through path170 defined.

Referring to FIGS. 11a) and b) there is illustrated, using the networktopology of FIG. 10b), an example of forwarding on the L1 cut-throughpath of FIG. 10b). L2 Forwarding in L1/L2/L3 network is accomplished byhaving:

Routers use L1 cut-through paths by installing ingress points to thepath as next hops in the IP Forwarding table.

Entries in the IP Forwarding table (IP prefixes) can be installed as:

Static routes. This is for the case where the L1 cut-through path isviewed as a link between two routers that is not part of the routingprotocol topology.

Forwarding Equivalence Class (FEC) elements. This is for the case wherethe L1 cut-through path is viewed as an MPLS LSP.

Before going out on the L1 cut-through path, the packet is placed intoan L2 frame.

This framing happens in all routers for the specific L2 which the packetis to be forwarded on.

The packet is also labelled with an MPLS label as is done for packetsbeing sent down an LSP.

In FIGS. 11a) and b) forwarding on a L1 cut-through path is illustrated.Packet 220 from RS 190 to RS 196 is sent to L1 cut-through path 170 andpasses through cross connects 226, 228, 232 and 236, but intermediateswitches do not perform label swapping or lookup. The use of the L1cut-through path thereby eliminates the L3 lookup of connectionlessforwarding, as well as the label swapping of L2 forwarding. The packetis unchanged during its transit over the L1 cut-through path.

Referring FIGS. 12a) and b) there is illustrated the router topology ofFIG. 10a) in which the cut-through path 170 has been broken by a faultcondition 250. When a physical link that is carrying multiple L1cut-through paths fails, each endpoint (R8 190 and R5 196 for L1cut-through path 170) of all the paths knows about the failure throughphysical detection methods specific to the cross connect technology.

In the preferred embodiment, an MPSL LSP is associated with every L1cut-through path, hence a router that detects an L1 cut-through pathfailure immediately informs the MPLS process that manages the LSPassociated with the path. The path failure causes an interrupt thatinforms the MPLS software process as soon as possible, of the failure.The router can then adjust the affected next hop fields in the L3forwarding table for the destination IP prefixes, which use the L1cut-through path, with other valid routes if they exist. This action cantake place more quickly at L2 than the L3 routing protocol reaction timeto the failed link because the detection method is based on L1 physicallayer detection that spans multiple cross connects. In L3 routingprotocols, link failure is propagated from the point of failure torouters farther and farther away. This means that a source router whichis far from the failure (many hops), some of whose traffic crossed thefailed link, does not find out about the failure for some time. Incontrast, the LSRs on the endpoints of L1 cut-through paths, which areaffected by a L1 link failure, are informed quickly even though they maybe several hops away from the failure.

If link 250 (R7-R6) fails, the router R8 190 immediately detects thefailure of L1 cut-through path 170 (R8-R7-R6-R5). Next hop entries, forexample in L3 forwarding table 222, which use the affected L1cut-through path, can be updated to not use the cut-through path 170.The router R8 190 could, for example, replace the next hop with L3connectionless next hop. That is, just send packets to R7 at L3.

Referring to FIG. 13 there is illustrated an L1/L2/L3 network inaccordance with an embodiment of the present invention. The L1/L2/L3network includes eight routers 190-204, all of which are MPLS capableand are thus Label Switching Routers (LSR). The L1/L2/L3 topology isaligned. L1 components could be SONET Ring, SONET link, TDM, or othersimilar technology. The following L1 cut-through paths are defined:R5-R4-R3, 252; R5-R4-R3-R2, 254; R5-R6-R1, 256; R4-R3-R2, 258. LSRs areconfigured with Strict Explicit Routed Label Switched Paths 262, 264,266, 268, that correspond to the L1 cut-through path, 252-258,respectively. Each LSR knows:

The existence of L1 cut-through paths that originate from it.

Path details for each originating L1 cut-through path, specifically therouters that it bypasses and the terminating LSR. For example, R5 196knows about three L1 cut-through paths 252, 254, 256 and theirconstituents R5-R4-R3, R5-R4-R3-R2, R5-R6-R1, respectively.

Summary knowledge of L1 cut-through paths is propagated through the L3network by the routing protocol. This includes only the endpoints andnot the intermediate nodes. For example, router 196, R5, knows about theR4->R2 cut-through path 258, but not the intermediate nodes of thatcut-through path.

For the network of FIG. 13, a backup router sequence (BRS) is defined tobe a node and link disjoint path for a given L1 cut-through path. Thisis done over the routing topology, which in this case is also the L1 andL2 topology. For each L1 cut-through path originating on it, an LSRcomputes or pre-computes a BRS. This can be done dynamically on each LSRin response to topology changes and L1 cut-through path changes. Anexample of a dynamic computation is to prune physical links andintermediate nodes of each L1 cut-through path, then run a shortest pathcalculation on the remaining topology. To be able to always have a BRS,there is a restriction on the network of FIG. 13. The L1 topology shouldbe engineered so that for any single link failure, all nodes remainconnected over some alternative path.

In operation when a L1 failure occurs, all L1 cut-through paths overthat link also fail. LSRs at the end points of those L1 cut-throughpaths detect this by L1 physical methods. For each failed L1 cut-throughpath, the LSR has a BRS. The LSR scans remaining L1 cut-through pathsthat originate from it to see if any of them have endpoints on the BRS.If so, the LSR can use any of them in constructing a new label switchedpath (LSP) which follow the BRS constituents. LSP setup procedures areused that are similar to those for explicit route (ER) setup with LDP,and follow the BRS from the L1 cut-through path endpoint to thedestination of the failed L1 cut-through path.

The LSP could also be constructed in advance, i.e. precomputed. That is,it is a backup LSP that is waiting to be used.

An L1 cut-through path can be selected whose endpoint is furthest in theBRS toward the destination LSR.

If there are no L1 cut-through path whose endpoints lie on the BRS, anER-LSP is setup following the BRS. The network's LSP could subsequentlybe re-optimized periodically if desired.

Referring to FIGS. 14a) and b) there are illustrated the networktopology of FIG. 13 with a failed link, and a node's instantaneoustopology database view after the failure, respectively. In the networkof FIG. 14a) a failure 270 has been introduced in link affecting L1cut-through path 252, 254, 258. The network topology as viewed by routerR5 196, is shown in FIG. 14b). Optical link R5-R3 fails. R5 196immediately detects loss of two L1 cut-through paths 252, 254. Failed L1cut-through paths are: (R5-R4-R3-R2) 254; (R5-R4-R3) 252; and (R4-R3-R2)258. The router R5 196 does not immediately know about loss of L1cut-through path (R4-R3-R2) 258 or link (R3-R4) as this is communicatedin the L3 routing protocol.

Referring to FIGS. 15a) and b) there are illustrated the networktopology of FIG. 14a) showing only the L1 cut-through path not affectedby the failed link and an LSP set up using the L1 cut-through path,respectively.

For L1 cut-through path (R5-R4-R3-R2) 254, its BRS is R5→R6→R1→R2. L1cut-through path (R5-R6-R1) 256 is on the BRS and is useable for aportion thereof. The router R5 196 establishes an LSP 272 overR5→(R5→R6→R1)→R1→R2 that is stacked over L1 cut-through path (R5-R6-R1)256.

Referring FIGS. 16a) and b) there is illustrated the router tables,label tables and cross-connects for the topology of FIG. 15b). Thebackup LSP 272 is now used in the IP Forwarding table 274 for packets276 whose destination is router R2 202. Label swapping occurs at therouter R1 204 using label table 280. Note how the L1 cut-through path(R5-R6-R1) 256 is used as the first hop in the backup LSP 272. Whencompared to a regular LSP setup over R5→R6→R1→R2, using the L1cut-through path 256 saves a label swap operation in the router R6.

In parallel with the failure sequence for the L1 cut-through path, theL3 routing protocol is updating the view of the topology throughflooding and SPF re-calculation. This eventually produces a stable viewof the topology at all LSR. Referring to FIG. 17 there is illustrated anL3 stabilized topology database view after the L3 routing protocol hasadjusted to the failure and updated the L3 routing tables throughout thenetwork.

Referring to FIG. 18 there is illustrated how a router recovers from thefailure of a second L1 cut-through path affected by the failure of FIG.14. R5 196 also handles the failure of L1 cut-through path (R5-R4-R3)252. Its recovery LSP 290 is R5→(R5→R6→R1)→R1→R2→R3

Because the two recovery LSPs 272 and 290 from R5 are separatelylabeled, they can co-exist over the same L1 cut-through path path 256that is used by their BRS. The router label table of R2 202 terminatesthe LSP 290 using the L1 cut-through path 256 (R5-R1-R2) and label swapsLSP R5-R1-R2-R3.

Referring to FIGS. 19a) and b) there is illustrated a network topologywhen the failed link recovers, L1 cut-through paths are automaticallyre-established by original configuration information, respectively. InFIG. 19a), when an LSR 196 (R5) sees a new L1 cut-through path (252 and254), it checks to see if there is an LSP (272 and 290) that originatesfrom the LSR 196 and could use the L1 cut-through path (252 and 254) asa full replacement. This LSP could be going over an existing L1cut-through path (as in the previous failure scenario). It could also bean LSP just using L3 links. The traffic flow is redirected over the L1cut-through path (252 and 290) after the SPF recalculates the forwardingtable and then the recovery LSP (272 or 290) is either torn down orremains alive but unused.

Recovering to a restored L1 cut-through path (e.g., 252) is exactly likemoving from a backup LSP (e.g., 290) to a primary LSP (e.g., 262).

Both LSP are valid entries in the IP Forwarding table, but the primaryLSP takes priority due to configured precedence.

The embodiments of the invention described herein above have thefollowing advantages:

Detection of a L1 link failure by LSRs that are not on either end of thelink is fast. That is, LSRs several hops away in the topology whose L1cut-through paths go across the failed link, are informed quickly of thefailure. This is relative to the speed at which an L3 routing protocolwould inform of the failure.

Use of the L1 cut-through path takes less processing at intermediatenodes than L3 or L2 forwarding.

After a failure detection, the forwarding table can be quickly adjustedto use a backup LSP.

This scheme “retrofits” static connections into the MPLS cut-throughpath forwarding mode, and thus enables existing MPLS configuration to beused for LSPs that overlay L1 cut-through paths.

It relies on fast connection failure detection and could apply to any L1network with this characteristic. For example:

SONET networks (rings, point-to-point links)

DWDM networks where L1 cut-through paths are wavelength channels

TDM networks where L1 cut-through paths are TDM paths.

An alternative embodiment has two L1/L2/L3 switches share the same crossconnect fabric, e.g., two routers attached to one SONET ADM.

In this case, the link between the two routers consists of one crossconnection as opposed to multiple ones in a path. There is no physicallink between the two LSRs and if the cross connect fabric itself fails,then this is treated like the failure of all L1 links attached to eitherLSR. Use of L1 cut-through paths with this switch embodiment works forfailure and recovery of other links in the network.

A Backup Router Sequence could be a link disjoint path only as opposedto a node and link disjoint. If so, then the BRS could be affected by anode failure in the steady state L1 cut-through path.

What is claimed is:
 1. A method of fault recovery for a networkincluding the steps of: establishing a physical topology for thenetwork; aligning a logical topology for the network with the physicalsuch that a router at a L1 cut-though path end point views a L1cut-through as a next hop; and using a fault indication from thephysical topology to effect fault recovery in the logical topology.
 2. Amethod as claimed in claim 1 wherein the step of establishing comprisesthe step of defining a node in the network as a combination of a crossconnect and a router.
 3. A method as claimed in claim 2 wherein the stepof establishing comprises the step of interconnecting network nodes viathe respective cross connects.
 4. A method as claimed in claim 3 whereinthe step of aligning includes a step of establishing a logical linkcorresponding each physical connection between nodes.
 5. A method asclaimed in claim 1 wherein the step of using includes the step ofdefining an alternative path and corresponding physical connection foreach primary route.
 6. A method as claimed in claim 5 wherein the stepof defining an alternative path includes the step of defining a layer 2link.
 7. A method as claimed in claim 6 wherein the layer 2 link is alabel switched path.
 8. A method as claimed in claim 7 wherein the labelswitched path is predetermined.
 9. A method as claimed in claim 7wherein the label switched path is defined at the time of the faultindication.
 10. Apparatus for data networking comprising: a crossconnect for switching at a physical layer; a router for redirecting datapackets at a logical layer coupled to the cross connect; and a faultrecovery mechanism responsive to a fault indication in the physicallayer for effecting a recovery in the logical layer.
 11. Apparatus asclaimed in claim 10 wherein the router includes an internetworkingprotocol (IP).
 12. Apparatus as claimed in claim 11 wherein the crossconnect is a synchronous optical network (SONET) add-drop multiplexor.13. Apparatus as claimed in claim 11 wherein the cross connect is a timedivision multiplex (TDM) cross connect.
 14. Apparatus as claimed inclaim 11 wherein the internetworking protocol includes layer 3 routing.15. Apparatus as claimed in claim 14 wherein the internetworkingprotocol includes layer 2 linking.
 16. Apparatus as claimed in claim 15wherein the internetworking protocol includes explicit route (ER)linking.
 17. Apparatus as claimed in claim 16 wherein theinternetworking protocol includes multi-protocol label switching (MPLS).18. A network comprising: a plurality of nodes, each node including across connect for switching at a physical layer, a router forredirecting data packets at a logical layer coupled to the cross connectand a fault recovery mechanism responsive to a fault indication in thephysical layer for effecting a recovery in the logical layer; aplurality of physical connections between nodes via the respective crossconnects; a plurality of logical routes between nodes via the respectiverouters; and an alternative logical route for use by the fault recoverymechanism.
 19. The network as claimed in claim 18 wherein the routerincludes an internetworking protocol (IP).
 20. The network as claimed inclaim 18 wherein the cross connect is a synchronous optical network(SONET) add-drop multiplexor.
 21. The network as claimed in claim 18wherein the cross connect is a time division multiplex (TDM) crossconnect.
 22. The network as claimed in claim 19 wherein theinternetworking protocol includes layer 3 routing.
 23. The network asclaimed in claim 22 wherein the internetworking protocol includes layer2 linking.
 24. The network as claimed in claim 23 wherein theinternetworking protocol includes explicit route (FR) linking.
 25. Thenetwork as claimed in claim 24 wherein the internetworking protocolincludes multi-protocol label switching (MPLS).
 26. In a networkincluding a plurality of nodes and having a plurality of communicationslayers, a method of providing fault recovery comprising the steps of:aligning at least a first and second layer of the plurality ofcommunications layers such that a router on the second layer at a L1cut-through path end point views a L1 cut-through of the first layer asa next hop; for a given path in the first layer, defining acorresponding path in the second layer and an alternative path in thesecond layer, the alternative path in the second layer corresponding toan alternative path in the first layer disjoint from the given path; andon detection in the first layer of a fault in the given path. switchingin the second layer from the corresponding path to the alternative path,whereby fault recovery in the network is provided in dependence uponspeed of detection in the first layer.
 27. A method as claimed in claim26 wherein the first layer path is a physical connection between twonodes in the network.
 28. A method as claimed in claim 27 wherein thephysical connection is a cut-through path spanning several nodes in thenetwork.
 29. A method as claimed in claim 28 wherein the cut-throughpath is viewed as a next hop by a third layer of the plurality ofcommunications layers.
 30. A method as claimed in claim 26 wherein thesecond layer path is a label switched path.
 31. A method as claimed inclaim 27 wherein end points of the second layer path correspond to endpoints of the cut-through path.
 32. A method as claimed in claim 26wherein the alternative first layer path includes a first layercut-through path between first and second nodes and a physicalconnection to a third node.
 33. A method as claimed in claim 32 whereinthe alternative second layer path uses the first layer cut-through pathas a first hop.
 34. A method as claimed in claim 26 wherein the givenfirst layer path and the corresponding second layer path are designatedas primary paths and the alternative first and second layer paths aredesignated as secondary paths.
 35. A method as claimed in claim 34wherein on the first designated path detecting that the fault no longerexists switching back to the primary paths.
 36. A method as claimed inclaim 26 wherein the step of switching in the second layer to providefault recovery is independent of fault recovery in a third layer of thenetwork.